1. Field of the Invention
The present invention relates to a photosensitive member having a photosensitive layer composed of a charge generating layer and a charge transporting layer. Particularly, the present invention relates to a photosensitive member having a thick photosensitive layer which is improved in sensitivity, repetition-properties and life.
2. Description of the Prior Art
In an electrophotographic system, copy images are formed by various kinds of methods. For example, the surface of a photosensitive member is charged and irradiated to form electrostatic latent images thereon, the electrostatic latent images are developed by a developer to be made visible and then the developed electrostatic latent images are fixed directly onto the photosensitive member (referred to as a direct method). In other method, developed electrostatic latent images on a photosensitive member which are made visible by a developer are transferred to a copy paper arid then, the transferred images are fixed on the paper (referred to as a powder transferring method). In another method, electrostatic latent images on a photosensitive member are transferred onto a copy paper, the transferred electrostatic latent images are developed by a developer and then fixed on the copy paper (referred to as an electrostatic latent image transferring method).
Known photosensitive materials for forming a photosensitive layer include inorganic photoconductive materials such as selenium, cadmium sulfide or zinc oxide.
These photoconductive materials have many advantages such as low loss of charges in the dark, an electrical charge which can be rapidly dissipated with irradiation of light and the like. However they have disadvantages. For example, a photosensitive member based on selenium is difficult to produce, has high production costs and is difficult to handle due to inadequate resistivity to heat or mechanical impact. A photosensitive member based on cadmium sulfide has defects such as its unstable sensitivity in highly humid environment and loss of stability with time because of the deterioration of dyestuffs, added as a sensitizer, by corona charge and fading with exposure.
Many kinds of organic photoconductive materials such as polyvinylcarbazole and the like have been proposed. These organic photoconductive materials have superior film-forming properties, are light in weight, etc., but inferior in sensitivity, durability and environmental stability compared with the aforementioned inorganic photoconductive materials.
Various studies and development have been in progress to overcome the above noted defects and problems. It has been proposed that charge generating function and charge transporting function are divided in a photosensitive layer, so that a function-divided photosensitive member which contains a charge generating material and a charge transporting material has been provided.
Such a function-divided photosensitive member has high productivity and low costs, since they can be prepared by a coating method, and a suitably selected charge generating material can freely control a region of photosensitive wavelength.
However such a function-divided photosensitive member has following problems; an initial surface potential is lowered, a residual potential increases and fogs are liable to be formed in copy images when used repeatedly. It is thought that these problems are brought about by a number of traps in a photosensitive layer, which are caused by interface conditions between a charge generating material and a binder resin or a charge transporting material and a binder resin, a barrier having an energy level, impurities contained in materials, deterioration of materials caused by corona discharge, exposure light, erasing light and the like, absorption of oxidizing gasses such as ozone and NOx, and deterioration of materials caused by the absorption. Therefore generated charges are thought to be trapped before they are combined with charges on surface.
Under the above circumstances, an aryl amine compound is proposed, because it has excellent performances as a charge transporting material, such as few traps, high mobility and low deterioration.
However mechanical properties are not satisfactory. There is limitation in durability. A photosensitive layer is worn under load such as friction with toner or paper, so that a thickness of a photosensitive layer decreases. Such a tendency is remarkable with respect to an organic photosensitive member. The decrease of layer thickness causes the lowering of chargeability. When the layer thickness becomes thinner than a specified thickness, the photosensitive member can not be put to use. Because an organic photosensitive member is poor in durability compared with an inorganic photosensitive member of selenium or amorphous silicone type, it has been applied to a low-speed copying machine. However, as it has been researched and developed recently that copying speed is made high, an organic photosensitive member for a high or middle speed copying machine is desired. The inventors have studied how to improve durability by use of excellent characteristics of an arylamine compound. It has been found that changes in electrical properties caused by wearing, in particular, lowering of chargeability can be prevented by making a photosensitive layer thick compared with a conventional layer thickness, in more detail, by making a charge transporting layer thicker.
However with respect to a conventional photosensitive member, when a charge transporting layer is made thicker, the number of traps increases in the charge transporting layer, so that residual potential is accumulated increasingly when used repeatedly.
In order to prevent above mentioned harmful influences, it has been studied that an electron-accepting compound is added into a charge transporting layer.
However unless a specified electron-accepting compound is used, a residual potential can not be controlled satisfactorily, and a surface potential and sensitivity are lowered after repetition use.
Further as an organic photosensitive member has been also applied to a laser printer and facsimile recently, higher image-reliability and repetition stability are required.